The use of liquid crystal arrays (LCA) as the display surface for a transparency has been suggested in the past. Such systems, in order to be optimally effective, require that the portion of the array which underlies the transparency (or the portion thereof) being viewed has sufficient brightness to enable viewing of the image and that the contrast ratio between the bright and dark portions of the array (namely the area outside the transparency or an image region of the transparency) be high.
The use of active matrix, passive matrix and direct drive technologies for LCAs have been described in the art. Each of these technologies, as they are used in the art, have substantial drawbacks when applied to a transparency viewer. Active matrix arrays have the drawback of high cost for the array because of the additional process steps required for the active elements and the difficulty in achieving large size arrays due to the limitations of semiconductor thin film technology.
Direct addressing systems have large spacings between the elements for the passage of the large number of conductors needed for this system, thus interrupting the continuity of the active regions of the LCA and reducing the contrast of the system. The large number of drivers required increases the cost of the associated electronics especially for large arrays.
Passive matrix systems have a much lower contrast and viewing angle than do active matrix systems because of the scanning limitations of the multiplex drive systems which are generally required for such systems. This is especially true when a number of different areas are bright and dark in systems which have the flexibility to provide a large number of such areas. On the other hand, both the passive matrix arrays and their associated electronics are more available and less expensive as compared to direct addressing systems. However, while such passive matrix systems do have smaller inactive stripes between the LCA elements than the direct addressing matrix LCAs, in some cases even these stripes can be troublesome to the viewer.
Another problem with LCA viewers is caused by the fact that the degree of contrast in LCAs is a strong function of the angle of incidence of the light which illuminates the LCA, with maximum contrast being available for near normal incidence. Since back illumination is by its nature relatively isotropic, the contrast of the viewing surface is further degraded.